1. Field of the Invention
This invention relates to a slit exposure shutter for a camera, such as a focal plane shutter, and more particularly to an improvement on a focal plane shutter of an electromagnetically driven type arranged to be operated by an electromagnetic force.
2. Description of the Prior Art
The advancement of application of electronics to cameras during recent years has resulted in the use of electronic circuits for light measurement, exposure control, information display, etc. However, in most cases, the shutter blades are still arranged to be driven by the force of a spring force which is charged by a winding up operation. Particularly, in the case of a shutter of a type requiring a relatively great driving force such as a focal plane shutter, a mechanical driving force is often employed in view of the limited capacity of the battery to be used. The focal plane shutter which is arranged to be operated by a mechanical driving force in this manner, however, must have a complex structure of the mechanism for driving the shutter screen. Such a complex structure is disadvantageous and, therefore, it has been a general desire to have an electromagnetically driven focal plane shutter having a simple structural arrangement for the driving mechanism.
Although an arrangement to drive a focal plane shutter with an electromagnetic force has been proposed in the past, a camera must be of a compact size for sufficient portability as generally known and this requirement necessitates the use of a small battery which does not have a sufficient capacity as power source for supplying a power to an electromagnetic device required for this purpose. While it is naturally more desirable that the electromagnetic device is of a large scale for a high efficiency in converting the electrical energy into a mechanical energy, the above stated requirement also necessitates the use of a small electromagnetic device which is of an inferior efficiency. Accordingly, compared with the output of the conventional mechanical, or spring type, shutter driving device, the electromagnetic shutter drive device has a small output. Besides, the use of a small battery the output of which fluctuates with temperature or the aging degree thereof causes the output of the electromagnetic device to be unstable. When a shutter screen travels, a frictional force is exerted either on the shutter screen itself or on the various parts cooperating therewith. This frictional force varies every time the shutter screen travels and also fluctuates with travelling points even within one travel.
When the front and rear screens of a focal plane shutter are driven independently of each other by means of the electromagnetic drive device which thus produces only a small and unstable output because of the above stated many reasons, the quantity of exposure light either varies every time an exposure is effected or comes to fluctuate with the points of exposure within a picture. This problem has been greatly hindering the practical applications of the conventionally known electromagnetically driven focal plane shutter.